The 8 iron IMHO should be used only when you can't reasonably choke a 9 or 7 iron enough. I use it well within it's range, never 100% anything and pray! There are lot's of threads on it and I made the same post as you, within a short while of getting the L90 R11 iron's, before I knew.
I will be 100% honest and say the PW needs a fuller understanding than I currently have as well, and I am working on that understanding still. The PW to me seems to to play shorter than you might expect uphill or into a wind......I think my mistake was too much BS with L81 Nikes, but it remain's work in progress for me...........Overall a great set though.......